How to connect a subwoofer into a mono block system

I have a Rel Storm III and run it off my mono blocks with fantastic results. I had to build a harness to connect to the amps which sit on the floor next to each speaker. That harness links to the speakon connector at a junction strip. It integrates perfectly with my speakers which run full-range.

So now, after discussions with REL tech support, and the very useful contributions above, I'm left with 2 options:
1. Use separate subs, one per mono block. This would mean from a financial and size standpoint using smaller subs. I'll need to stay under $450/sub. This would acoustically bring in the amp signature, a positive.
2. Using the preamp outputs directly to a single sub. My AI M3B is a great preamp, and I'll need to prevent bass doubling through controls on the sub. But this option allows a larger sub, albeit a single.

In all a decision like many driven by economics, aesthetics and some ignorance.

If you can afford it, use two subs. I've been thru two REL subs. The Storm II and Stentor. They are really good with music. I needed two subs to even out bass response and "pressurize the room"! I ended up with a pair of JL Audio subs. If you adjust the subwoofer correctly, it fills the room properly. Bass isn't soft at a live event, it makes its presence known. Ever hear a kick drum or Timpani live?

The "pressurized room" thing actually is the "natural" state of sound and should be respected as a matter of degree of course...the low frequencies you hear as a part of the sort of "room sound" in live music anywhere seems to be the mojo missing from systems lacking a full range. "Pressurized" just sounds dangerous, but unless you get crazy with 18" driver multi thousand watt subs in your converted shipping container listening room, you should be safe.